Influence of catheter materials and tissue composition on low dose rate iridium-192 seed implant dosimetry

To quantify the perturbing influence of catheter materials and tissue composition on the dose distribution in water around a low dose rate, steel encapsulated Ir-192 seed.

The EGS4 Monte Carlo code was used to estimate point doses in a bounded phantom within a cylinder with a radius of 3 cm surrounding the source. Simulations were performed in water and soft tissue media for an unadorned seed, and then in soft tissue for a nylon-ribbon-encased seed inside steel and nylon catheters. The latter simulations were repeated including inactive seeds on either side of the source at spacings of 1.0 and 0.5 cm. Sufficient numbers of photon histories were run to reduce the relative statistical uncertainty of most dose estimates to <0.1%. Contributions from primary and scattered photons were scored separately.

For the unadorned source, doses in soft tissue were uniformly approximately 1% lower than in water. Introduction of either catheter perturbed the dose in soft tissue by <0.4% along the transverse source axis, r. In the long axis direction, d, dose in soft tissue for the steel catheter relative to the unadorned source fell with increasing distance and decreasing radius, r, and trended oppositely for the nylon catheter. Neighboring seeds did not affect the dose distribution except at r = 0.2 cm for seeds spaced at 0.5 cm, where dose fell approximately 8% in the interval d = 4-7 cm. For several Paris system implant geometries, the presence of catheters altered basal doses in a systematic manner.

Perturbations of the dose distribution for LDR Ir-192 seeds in water introduced by treatment catheters and tissue composition are small but systematic. If desired, they can be accounted for in clinical volumes of interest by simple adjustment of dose calculation parameters.